Metallic species sintered product of columbium or tantalum with ferromagnetic iron



United States This invention relates to a metallic species product and, more particularly, to such a product having physical, mechanical and electrical properties acceptable to the testing elements for silver-base coins in a coin-operated machine.

The present shortage of silver for species and com mercial usage, and the contemplated future aggravation of this shortage, has led to an intensive search for a substitute for the silver-base alloys now used in coins. An acceptable substitute material in coin form must not only have an intrinsic value close to, but not above, the face value of the coin but must be readily available. It must also have physical and mechanical properties, such as density, hardness and resilience, which are acceptable to the testing elements of coin-operated machines. In addition, the trend toward future coining by means of powder metallurgy techniques dictates the further requirement that the substitute material be amenable to these techniques. High purity columbium and tantalum powders possess characteristics which make it possible to produce species coins meeting the aforementioned specifications.

However, the testing system of coin-operated machines includes an electrical testing element responsive to the outstandingly low resistivity of the silver-copper alloy used for silver coins, and columbium and tantalum have a resistivity so much higherthan that of silver coins as to be consistently rejected by the electrical testing element. This can be readily appreciated from the fact that the 90% silver-% copper alloy used for silver coins has a resistivity of about 2.0 micro-ohms per centimeter, whereas the resistivity of columbium is about 14.6 and of tantalum is about 13.6. In the electrical testing element of a coin-operated machine, the coin, after passing mechanical tests for size, density and resilience, slides downwardly along a sloping ramp through a magnetic field. The low resistivity of a silver coin permits the generation of relatively high current in the coin as it passes through this magnetic field, and the current thus induced in the coin causes its descent to be slowed sufficiently to avoid passing into the reject system of the machine. Coins made of columbium or tantalum, or both, as well as alloys based on these metals, have such high resistivity that they are not appreciably slowed as they pass through the magnetic field of the testing element and consequently are rejected.

We have now discovered that when coins are produced from columbium and tantalum powders by the pressing and sintering operations of conventional powder metallurgy techniques, they can be given a magnetic characteristic which results in their being retarded as they move through the magnetic field of the coin-testing element to the same degree as if the coin had the low resistivity of a silver coin. This result is achieved pursuant to the present invention by incorporating in the columbium or tantalum powder an extraneous ferromagnetic powder which, under the pressing and sintering conditions used to mint the coins, remains free from dissolution in the columbium or tantalum to an extent suflicient to impart an appropriate magnetic characteristic to the coin to be accepted by the electrical testing element of coin-operated machines.

atent The columbium and tantalum powders useful for this purpose should be of relatively high purity so that they are sufficiently ductile to be compacted by compression force of the magnitude conventionally used in powder metallurgical operations. For this purpose a purity of at least about 99.9% is presently desirable, but it will be readily appreciated that somewhat less pure powders can be used if suitably high compaction pressure is available. It has also been found that the shape of the powder particles has an influence on the compactability of the powder, irregularly shaped particles being more amenable to compaction than round or other regularly-shaped particles. columbium and tantalum powders obtained by sodium reduction of potassium columbium fluoride or potassium tantalum fluoride, respectively, have the desired purity and irregular shape for ease of compaction and have particle sizes of minus mesh Tyler Standard which make them particularly suitable for this purpose.

The ferromagnetic material useful in practicing the invention should be a metal (this term being used herein and in the claims to include alloys) which is not readily dissolved or otherwise assimilated by the columbium or tantalum powder during pressing and sintering. For this purpose, iron powder is presently preferred. However, other ferromagnetic metals such as nickel and cobalt, and alloys of these elements with one another and with iron, can be used effectively. The present preference for iron powder is based on the fact that it does not significantly dissolve in the columbium or tantalum during pressing and sintering, whereas nickel does partially dissolve and must therefore be .used in greater amount so as to provide in the pressed and sintered product an amount of nickel free from such dissolution sufficient to impart the desired magnetic properties to the product. Ferromagnetic materials other than metals, such for example as ferrosoferric oxide, are not suitable for the practice of the invention because they react with the columbium or tantalum with resulting degradation of the latter.

The amount of ferromagnetic metal powder incorporated in the powder mass consisting predominantly of columbium or tantalum, or mixtures thereof, depends on the magnetic properties of the ferromagnetic metal, on the extent to which this metal is sequestered by the columbium or tantalum and on the magnetic properties of the final product required to pass the electrical testing element of a coin-operated machine. In order to be acceptable to the testing element of present machines designed to accept silver dimes and quarters, an amount of 1% to 5% by weight of substantially pure iron powder will impart to columbium powder, when admixed therewith, pressed and sintered, the ability to be accepted by these testing elements. However, the amount of magnetic material required is further affected by the density of the compaction and by the extent of sintering of the compacted mass, more magnetic material being required by pressing and sintering conditions which result in higher densities of the final product. Accordingly, the amount of ferromagnetic metal incorporated in the columbium or tantalum powder will vary with variations in the conditions set forth hereinbefore, but the optimum amount required to produce the desired result can be readily ascertained by one skilled in the art.

Compaction of the powder mix is effected in conventional equipment used for powder metallurgy. Compaction pressure will generally be above about 25,000 p.s.i., and it is presently preferred to use a compaction pressure of at least 50,000 p.s.i.

Sintering of the compacted powder mix is carried out at temperatures generally within the range of about 1200 to 2000 C. The optimum sintering time at temperature varies with the temperature used, the extent of the sintering required and the density desired in the final product. For example, when sintering at about 17001800 C., sufficient sintering can be achieved in the time required merely to bring the compact up to final temperature, although sintering at final temperature for about to minute-s can be used advantageously, particularly when using sintering temperatures in the lower portion of the aforementioned range. The sintering atmosphere should be inert with respect to the components of the compact, and for this purpose vacuum sintering as well as sintering in an inert atmosphere of argon or helium produces a desirable final product.

The following example is representative of the production of a columbium-base product of the present invention. The columbium powder was obtained by sodium reduction of potassium columbium fluoride and had an irregular particle shape of a size of minus mesh, through mesh. The purity of the columbium was 99.9%. To this powder there was added 2% by weight of electrolytic iron powder having a particle size of minus 200 mesh. The two powders were thoroughly mixed and were then compacted into coinshaped masses with a molding pressure of about 25,000 psi. The compacts were then vacuum sintered by bringing them up to a final temperature of about 1200 C. and holding them at this temperature for about 1 minute. They were then permitted to cool to about ambient temperature. These conditions produced a lightly sintered product of about 70% density, i.e. a density of about 6.0 gr. per cc. The iron powder was not significantly dissolved by the columbium as evidenced by the fact that the sintered product was magnetic. The sintered coin-shaped compacts were inserted in a conventional coin-operated machine and were consistently accepted by the physical, mechanical and electrical testing elements of the machine.

We claim:

1. A metallic species product having physical, mechanical and electrical properties such as to be acceptable to the testing elements for silver-base coins in a coin-operated machine, said product consisting essentially of a powder-metallurgically pressed and sintered mixture of a major proportion of a metal pbwder of the group consisting of columbium and tantalum powder and a minor proportion of ferromagnetic metal powder free from dissolution in the columbium or tantalum.

2. A metallic species product having physical, mechanical and electrical properties such as to be acceptable to the testing elements for silver-base coins in a coin-operated machine, said product consisting essentially of a powder-metallurgically pressed and sintered mixture of a major proportion of columbium powder and a minor proportion of ferromagnetic metal powder free from dissolution in the columbium.

3. A metallic species product having physical, mechanical and electrical properties such as to be acceptable to the testing elements for silver-base coins in a coin-operated machine, said product consisting essentially of a powder-metallurgically pressed and sintered mixture of a metal powder of the group consisting of columbium and tantalum powder and a minor proportion of iron powder free from dissolution in the columbium or tantalum.

4. A metallic species product having physical, mechanical and electrical properties such as to be acceptable to the testing elements for silver-base coin-s in a coinoperated machine, said product consisting essentially of a powder-metallurgically pressed and sintered mixture of columbium powder and about 2% by weight of iron powder free from dissolution in the columbium.

5. A metallic species product having physical, mechanical and electrical properties such as to be acceptable to the testing elements for silver-base coins in a coinoperated machine, said product consisting essentially of a powder-metallurgically pressed and sintered mixture of high purity columbium powder and about 2% by weight of ferromagnetic iron powder free from dissolution in the columbium.

6. A metallic species product having physical, mechanical and electrical properties such as to be acceptable to the testing elements for silver-base coins in a coinoperated machine, said product consisting essentially of a powder-metallurgically pressed and sintered mixture of high purity columbium powder and 1% to 5% by weight of ferromagnetic iron powder free from dissolution in the columbium.

No references cited.

LEON D. ROSDOL, Primary Examiner.

R. L. GRUDZIECKI, Assistant Examiner. 

1. A METALLIC SPECIES PRODUCT HAVING PHYSICAL, MECHANICAL AND ELECTRICAL PROPERTIES SUCH AS TO BE ACCEPTABLE TO THE TESTING ELEMENTS FOR SILVER-BASE COINS IN A COIN-OPERATED MACHINE, SAID PRODUCT CONSISTING ESSENTIALLY OF A POWDER-METALLURGICALLY PRESSED AND SINTERED MIXTURE OF A MAJOR PROPORTION OF A METAL POWDER OF THE GROUP CONSISTING OF COLUMBIUM AND TANTALUM POWDER AND A MINOR PROPORTION OF FERROMAGNETIC METAL POWDER FREE FROM DISSOLUTION IN THE COLUMBIUM OR TANTALUM. 